im a pretty small guy and need some tips on how to take out a 250 pounder.i dont fight that much but ive got a good punch and i know im in much better shape than he is, what are some good ways to turn his size into a disadvantage?

Taking on someone bigger is always a challenge. Perhaps some grappling techniques (if that's permitted in your competition) and circle him so he can't charge you easily. A little more information about the format of the competition would help people here answer you better.